1. Field of the Invention
The present invention relates generally to a CDMA communication system, and in particular, to a device and method for generating and distributing coded symbols capable of preventing degradation in channel performance during data transmission.
2. Description of the Related Art
Code Division Multiple Access (CDMA) communication systems are implemented according to the IS-95 standard. With an increase in the sophistication of CDMA communication technology and a decrease in usage costs, there has been an exponential growth in the number of subscribers of CDMA communication services. Accordingly, many methods have been proposed for meeting subscribers"" ever-increasing demands for high quality CDMA service. For example, several methods for improving a forward link structure in the CDMA communication system have been proposed.
One such method for improving the forward link structure, especially a forward link fundamental channel designed for a third generation multicarrier CDMA system, was proposed in the TIA/EIA TR45.5 conference and approved on May 15, 1998 by the Telecommunications Industry Association (TIA). A forward link structure for a multicarrier CDMA communication system is illustrated in FIG. 1.
With reference to FIG. 1, a channel encoder 10 encodes input data, and a rate matcher 20 repeats and punctures symbols outputted from the channel encoder 10. Here, the data input to the channel encoder 10 has a variable bit rate. The rate matcher 20 repeats and punctures the coded data bits (i.e., symbols) outputted from the channel encoder 10 to match symbol rates for the data having the variable bit rate. A channel interleaver 30 interleaves an output of the rate matcher 20. A block interleaver is typically used for the interleaver 30.
A long code generator 91 generates a long code which is identical to that used by the subscriber. The long code is a unique identification code for the subscriber. Thus, different long codes are assigned to the respective subscribers. A decimator 92 decimates the long code to match a rate of the long code to a rate of the symbols outputted from the interleaver 30. An adder 93 adds an output of the channel interleaver 30 and an output of the decimator 92. An exclusive OR gate is typically used for the adder 93.
A demultiplexer 40 sequentially multiplexes data outputted from the adder 93 to multiple carriers A, B and C. First to third binary-to-four level converters 51-53 convert signal levels of binary data outputted from the demultiplexer 40 by converting input data of xe2x80x9c0xe2x80x9d to xe2x80x9c+1xe2x80x9d and input data of xe2x80x9c1xe2x80x9d to xe2x80x9cxe2x88x921xe2x80x9d. First to third orthogonal modulators 61-63 encode data outputted from the first to third level converts 51-53 with corresponding Walsh codes, respectively. Here, the Walsh codes have a length of 256 bits. First to third spreaders 71-73 spread outputs of the first to third orthogonal modulators 61-63, respectively. Here, QPSK (Quadrature Phase Shift Keying) spreaders can be used for the spreaders 71-73. First to third attenuators (or gain controllers) 81-83 control gains of the spread signals outputted from the first to third spreaders 71-73 according to corresponding attenuation signals GA-GC, respectively. Here, the signals outputted from the first to third attenuators 81-83 become different carriers A, B and C.
In the forward link structure of FIG. 1, the channel encoder 10, having a coding rate of R=1/3, encodes the input data into 3 coded data bits (i.e., code words or symbols) per bit. Such coded data bits are demultiplexed to the three carriers A, B and C after rate matching and channel interleaving. The multicarrier CDMA communication system of FIG. 1 can be modified to a single carrier CDMA communication system by removing the demultiplexer 40 and using only one level converter, one orthogonal modulator, one spreader and one attenuator.
FIG. 2 is a detailed diagram illustrating the channel encoder 10, the rate matcher 20 and the channel interleaver 30. In FIG. 2, data of a first rate is composed of 172 bits (fall rate) per 20 ms frame; data of a second rate is composed of 80 bits (xc2xd rate) per 20 ms frame; data of a third rate is composed of 40 bits (xc2xc rate) per 20 ms frame; and data of a fourth rate is composed of 16 bits (xe2x85x9 rate) per 20 ms frame.
First to fourth CRC generators 111-114 generate CRC bits corresponding to the respective input data having different rates and add the generated CRC bits to the input data. Specifically, 12-bit CRC is added to the 172-bit data of the first rate; 8-bit CRC is added to the 80-bit data of the second rate; 6-bit CRC is added to the 40-bit data of the third rate; and 6-bit CRC is added to the 16-bit data of the fourth rate. First to fourth tail bit generators 121-124 add 8 tail bits to the CRC-added data, respectively. Therefore, the first tail bit generator 121 outputs 192 bits; the second tail bit generator 122 outputs 96 bits; the third tail bit generator 123 outputs 54 bits; and the fourth tail bit generator 124 outputs 30 bits.
First to fourth encoders 11-14 encode data output from the first to fourth tail bit generators 121-124, respectively. A convolutional encoder having a constraint length of K=9 and a coding rate of R=1/3 can be used for the encoders 11-14. In this case, the first encoder 11 encodes the 192-bit data output from the first tail bit generator 121 into 576 symbols of full rate; the second encoder 12 encodes the 96-bit data output from the second tail bit generator 122 into 288 symbols of xc2xd rate; the third encoder 13 encodes the 54-bit data output from the third tail bit generator 123 into 162 symbols of about xc2xc rate; and the fourth encoder 14 encodes the 30-bit data output from the fourth tail bit generator 124 into 90 symbols of about xe2x85x9 rate.
The rate matcher 20 includes repeaters 22-24 and symbol deletion devices 27-28. The repeaters 22-24 repeat symbols outputted from the second to fourth encoders 12-14 according to predetermined times to increase output symbol rates thereof to the fall rate. The symbol deletion devices 27 and 28 delete symbols outputted from the repeaters 23 and 24 which exceed the symbols of the full rate in number. Since the second encoder 12 outputs 288 symbols which is xc2xd the 576 symbols outputted from the first encoder 11, the second repeater 22 repeats the received 288 symbols two times to output 576 symbols. Further, since the third encoder 13 outputs 162 symbols which is about xc2xc the 576 symbols outputted from the first encoder 11 , the third repeater 23 repeats the received 162 symbols four times to output 648 symbols which exceeds the 576 symbols of full rate in number. To match the symbol rate to the full rate, the symbol deletion device 27 deletes every ninth symbol to output 576 symbols of full rate. In addition, since the fourth encoder 14 outputs 90 symbols which is about xe2x85x9 the 576 symbols output from the first encoder 11, the fourth repeater 24 repeats the received 90 symbols eight times to output 720 symbols which exceeds the 576 symbols of full rate in number. To match the symbol rate to the full rate, the symbol deletion device 28 deletes every fifth symbol to output 576 symbols of full rate.
First to fourth channel interleavers 31-34 interleave the symbols of full rate outputted from the first encoder 11, the second repeater 22, the symbol deletion device 27 and the symbol deletion device 28, respectively. Forward Error Correction (FEC) is used to maintain a sufficiently low Bit Error Rate (BER) of a mobile station for a channel having a low signal-to-noise ratio (SNR) by providing a channel coding gain. The forward link for the multicarrier communication system can share the same frequency band with the forward link for the IS-95 system in an overlay method. However, the overlay method provides the following problems.
In the overlay method, three forward link carriers for the multicarrier system are overlaid on three 1.25 MHz bands used in the IS-95 CDMA system. FIG. 3 illustrates transmission power levels, by the respective bands, of base stations for the IS-95 system and the multicarrier system. In the overlay method, since the frequency bands for the multicarrier system are overlaid on the frequency bands for the IS-95 system, the transmission power or channel capacity is shared between the IS-95 base station and the multicarrier base station at the same frequency band. In the case where the transmission power is shared between the two systems, the transmission power is first allocated for the IS-95 channel which mainly supports a voice service and then, the maximum transmission power permissible to the respective carriers for the multicarrier CDMA system is determined. Here, the maximum transmission power cannot exceed a predetermine power level, because the base station has a limited transmission power. Further, when the base station transmits data to many subscribers, interference among the subscribers increases resulting in an increase in noise. FIG. 3 illustrates the state where the IS-95 base station and the multicarrier base station allocate substantially equal transmission power at the respective 1.25 MHz frequency bands.
However, the IS-95 channels of 1.25 MHz frequency bands have a different transmission power according to a change in the number of subscribers in service and a change in voice activity of the subscribers. FIGS. 4 and 5 illustrate situations where the transmission power allocated for the multicarrier base station decreases at some carriers, as the transmission power allocated for the IS-95 base station increases rapidly at the corresponding frequency bands due to an increase in the number of IS-95 subscribers. As a result, sufficient transmission power cannot be allocated for one or more of the multiple carriers so the SNRs are different according to the carriers at the receiver. Accordingly, a signal received at a carrier having the low SNR increases in a bit error rate (BER). That is, when the number of IS-95 subscribers increases and the voice activity is relatively high, a signal transmitted via a carrier overlaid on the corresponding frequency band increases in the BER, resulting in a decreased system capacity and an increased interference among the IS-95 subscribers. That is, the overlay method may cause a reduction in capacity of the multicarrier system and an increase in interference among the IS-95 subscribers.
In the multicarrier system, the respective carriers may have independent transmission powers as illustrated in FIGS. 4 and 5. With respect to performance, FIG. 4 shows the power distribution being similar to the case where a R=1/2 channel encoder is used, and FIG. 5 shows the power distribution being worse than the case where the channel encoder is not used. In these cases, one or two of the three coded bits (i.e., symbols) for an input data bit may not be transmitted, causing a degradation in system performance.
Therefore, even in a direct spreading CDMA communication system using a single carrier, weight distribution of the symbols generated by channel encoding is poor, thereby causing a degradation in channel decoding performance.
It is, therefore, an object of the present invention to provide a channel encoding device and method capable of generating coded data exhibiting a good channel coding performance in a CDMA communication system.
It is another object of the present invention to provide a channel encoding device and method capable of generating channel coded data having a good channel coding performance and effectively distributing the generated channel-coded data to respective carriers in a multicarrier CDMA communication system.
It is further another object of the present invention to provide a channel transmission device and method for distributing generated symbols to carriers such that system degradation caused by symbols damaged during transmission is minimized in a multicarrier CDMA communication system.
It is still another object of the present invention to provide an R=1/6 convolutional encoding device and method capable of increasing channel performance of a channel transmitter in a CDMA communication system.
To achieve the above objects, there is provided a communication system using at least two carriers. The communication system includes a channel encoder for encoding data, a channel controller for generating a control signal for transmitting channel coded symbols for performing decoding using data received via at least one carrier, and a symbol distributer for assigning the channel coded symbols to at least two carriers.
Also, there is provided a channel encoding device having a plurality of delays for delaying an input data bit to generate first to eight delayed data bits; a first operator for exclusively Oring the input data bit and the third, fifth, sixth, seventh and eighth delayed data bits to generate a first sysmbol; a second operator for exclusively Oring the input data bit and the first, second, third, fifth, sixth and eighth delayed data bits to generate a second symbol; a third operator for exclusively Oring the input data bit and the second, third, fifth and eighth delayed data bits to generate a third symbol; a fourth operator for exclusively Oring the input data bit and the first, fourth, fifth, sixth, seventh and eighth delayed data bits to generate a fourth symbol; a fifth operator for exclusively Oring the input data bit and the first, fourth, sixth and eighth delayed data bitss to generate a fifth symbol; and a sixth operator for exclusively Oring the input data bit and the first, second, fourth, sixth, seventh and eighth delayed data bits to generate a sixth symbol.